This invention relates to a process for the production of a magnetic recording medium by electroless plating and more particularly, it is concerned with a novel process for the production of a magnetic recording medium excellent in adhesiveness as well as in surface property by electroless plating.
Thin layers of ferromagnetic metals formed by electroplating, electroless plating, sputtering, vacuum vapor deposition or iron plating have lately been noticed as a binder-free, so called non-binder type, magnetic recording medium in place of the magnetic recording media of the prior art in which a powdered magnetic material of .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, CrO.sub.2 or ferromagnetic alloy powders is dispersed in an organic binder and coated. It has been proposed theoretically and experimentally to raise the coercive force as well as to make the thickness thinner as one requirement for magnetic recording media for high density recording and it has been expected that such a non-binder type magnetic recording medium has great possibilities, which can readily be made thinner by a factor of 10 than coated type magnetic recording media and has a very large saturated magnetization.
The electroless plating method has an advantage that a magnetic layer can be formed on, in particular, a non-conductive substrate with a good adhesiveness and a magnetic layer having excellent magnetic properties can uniformly be formed. In this specification, the electroless plating means a chemical reduction plating wherein a metal ion to be plated is reduced with a reducing agent in a plating solution and deposited on a substrate under metallic state. In order to start and accelerate this reduction reaction on the surface of the substrate only, it is necessary to give a catalytic activity to the surface of the substrate by a pretreatment. Where a plastic substrate is subjected to magnetic plating, for example, various steps of defatting-etching-substrate surface activating-electroless magnetic plating have commonly been carried as disclosed in U.S. Pat. Nos. 3,245,826 and 3,353,986. The steps of defatting and etching are carried out so as to form a magnetic plating layer uniformly on a substrate with a good adhesiveness and both steps may be accomplished by one solution. For example, an aqueous solution of sodium hydroxide or mixed solution of sulfuric acid and a bichromate is widely used as a defatting and etching solution. The usual method for the substrate surface activation treatment consists in sensitization using a sensitizer consisting of a hydrochloric-acidic solution of SnCl.sub.2 and subsequent activation using an activator containing a noble metal ion such as Pd, Au or Ag, as described in U.S. Pat. No. 2,702,253. On the surface of the substrate thus treated are bonded fine particles of Pd, Au or Ag capable of acting as a catalyst of plating reaction. The other methods for the substrate surface activation treatment, as described in U.S. Pat. Nos. 3,011,920 and 3,532,516, have also been put to practical use, which consist in a treatment with a Pd-Sn sol (catalyst treatment) and a subsequent treatment with an accelerator.
As an electroless magnetic plating bath to obtain a high saturation magnetization and coercive force, well known are baths containing cobalt ion or cobalt ion plus nickel ion as a magnetic metal ion and hypophosphite ion as a reducing agent (U.S. Pat. Nos. 3,116,159 and 3,219,471). A magnetic recording medium having a magnetic layer formed by electroless plating is suitable for short wavelength recording, in particular, which recording wavelength is approximately 1 .mu.m like a video signal, but the practical problems such as stripping of a magnetic plating layer during friction with a head due to its low adhesiveness, and lack of the reproducing output due to the surface irregularity, cannot completely be solved in the magnetic recording medium obtained by the prior art electroless plating method. In the case of producing a magnetic recording medium by forming a magnetic layer on a plastic substrate by electroless plating as described above, it has been proposed to treat the surface of the plastic substrate with an aqueous solution of caustic soda or mixed solution of dichromate and sulfuric acid so as to improve the adhesiveness of the magnetic plating layer and to effect an even plating. If, however, the extent of this surface treatment is large, the surface property of the magnetic plating layer is deteriorated, although the adhesiveness is improved; while if the extent of this surface treatment is small, the surface smoothness is improved, but the adhesiveness is not increased. Therefore, a magnetic recording medium whose adhesiveness and surface property are simultaneously improved is hardly obtained.
We, the inventors, have made various studies on a process for the production of a magnetic recording medium by electroless plating and consequently found that a magnetic recording medium excellent in adhesiveness as well as in surface property can be produced by subjecting the surface of a plastic substrate to a glow discharge treatment and then to an electroless plating to form a magnetic thin layer.